The most common form of centrifugal compressor that is used with internal combustion engines is a turbocharger wherein the impeller is driven by a turbine powered by the engine exhaust gases. This works well for a 4-cycle engine where the nominal combustion air is drawn into the cylinders on the piston downstroke. But in an externally scavenged 2-cycle engine there is no separate intake and exhaust stroke and instead there is a combined stroke wherein the combustion residuals must be pushed out of the cylinders with the fresh air for the next combustion event. In such an application, the air compressor is the sole source of combustion/scavenge air and therefore at start up, there is insufficient exhaust gas flow to spin the turbine of a turbocharger with the result that there would be no combustion air for starting the engine. And thus a turbocharger is not suitable for such an application. Mechanically driving the compressor eliminates this start-up problem on a 2-cycle engine and can also be used in a 4-cycle engine. However, there is considerable difficulty in providing a suitable mechanical drive that will produce the necessary high compressor impeller speeds in a highly reliable manner and without objectionable noise levels. Moreover, it should be capable of being mass produced at a reasonable cost.
The mechanical drive between the engine crankshaft and the compressor impeller must provide a very high overdrive (output/input) speed ratio in order to drive the impeller at the extremely high speeds necessary to obtain acceptable operating efficiencies. For example, impeller speeds exceeding 100,000 RPM at peak engine speed are not uncommon and the overdrive speed ratio necessary to obtain same typically exceeds 15:1 and will of course vary with the relevant operating characteristics of both the engine and the compressor. Examples of mechanical drives that have been proposed for centrifugal air compressors are disclosed in U.S. Pat. Nos. 1,754,724; 1,852,569; 2,099,785; 2,294,743; and 5,063,904. As illustrated by these patents, such mechanical drives typically include various forms of gearing such as planetary gear drives and spur gear trains that may include an internal ring gear and pinion gear. Then there is also my U.S. Pat. No. 5,058,546 that discloses a mechanical drive employing a belt and pulley drive with a low overdrive speed ratio from the engine crankshaft in series with a friction drum and wheel providing a high overdrive speed ratio drive to the compressor impeller. The problems with an all gear drive include the noise of the gears and their attendant cost. And the gears, while providing a desirable positive type of drive, require a special mounting of the compressor gear train to include the engine crankshaft. On the other hand, a belt and pulley drive allows the centrifugal air compressor to be installed at various locations on the engine like other belt driven engine accessories. Moreover, the belt may be one that also drives one or more of the other engine accessories which is advantageous from both a cost standpoint and space allocation. But the belt and pulley drive must be limited to a low overdrive speed ratio of about 2:1 to prevent the belt from slipping on the smaller pulley at the high speed/load conditions encountered in such an application. As a result, a belt and pulley drive must be combined with a mechanical drive in some form that provides a high overdrive ratio that is highly reliable but yet does not produce objectionable noise levels and can be mass produced at a reasonable cost. In my above patent, such a high overdrive ratio is provided by a friction wheel and drum and while this has proven satisfactory and particularly with respect to noise, a geared overdrive has the potential for more load capacity and even greater reliability provided that the gear noise can be held to an acceptable level, the remainder of the compressor components are also highly reliable, and the cost is not prohibitive.